In the trough mangles existing on the market the padded hollow roller is encompassed by the bearing surfaces over an arc of its circumference of at least 165.degree.. During the drying of the laundry, the resulting steam is evacuated by a frontally attached blower through the roller wrapping as well as through a multitude of holes in the steel wall of the hollow roller. These blowers have to be arranged to handle the steam generated by a maximum mangling load. Since the blowers are dimensioned with a safety factor with relation to the delivery (negative pressure) and discharge, even with maximum mangling loads a negative pressure can prevail inside the hollow roller (pressing roller) which results in the intake of cool room air through the free surface of the roller wrapping not covered by the bearing surfaces. The free surface comprises approximately 195.degree. of arc of the roller wall.
If the mangling is performed with little laundry or with interruptions when there is no laundry to be pressed, then, due to the exhaust resulting from the rotational speed of the hollow roller, energy is wasted. In this case, air at room temperature is drawn in through the turning hollow roller, cools it on its way to discharge from the pressing trough to the reentry and the drying speed is reduced by approximately 10-15% while additional energy in the form of steam must be fed to the pressing trough as required for heating this air.
It is therefore required for 1 kg laundry having a normal residual moisture at the mangling loading rate of 40 to 60% which is common in practice to provide an amount of steam of 1.2 kg or more for heating the mangle. The thermal efficiency is consequently in the range of 50% or less.
Numerous proposals aimed at reducing the energy consumption in such common mangles have been published. For instance it has been proposed to cover the roller wrapping with a shell extending at a distance of approximately 3 cm from the surface of the roller wrapping and parallel thereto. This can affect only a slight reduction in energy consumption because the heating of the room air at the hot surfaces on the upper side of the mangle heat radiation are reduced.
Due to the fact that the shell is positioned at a distance from the roller wrapping it is possible for equal amounts of the room air, as in the fully open pressing roller, to be drawn in through the larger slots at the entrance and exit ends having a cross section of approximately 1 sqm. This room air will then be unnecessarily blown out as hot air. Since the cross section of the suction connection on the pressing roller is considerably smaller than the cross section of the mentioned slots, the surface of the pressing roller is disadvantageously provided with room air.
The German open application No. 19 37 738 tries to eliminate this disadvantage proposing that the feed- and discharge-end rims of a shell shaped as a hood with reflector be sealed opposite to the roller wrapping. Since the steam generated during mangling must be exhausted from the drum axially, the construction of heat-resistant bearings of a larger diameter and the high cost related thereto are unavoidable. The sealing of the hood against the roller wrapping leads to work interruptions and the related drops in temperature to corrosion, requiring considerable added expenses for adequate countermeasures to avoid that.